ISTANBUL — The suicide bomber who carried out an attack in Istanbul’s central tourist district had entered Turkey from Syria and was registered as a refugee, Turkish officials said Wednesday, adding to concerns over security at the country’s critical border.

The bomber, who died in the attack on Tuesday, was identified as Nabil Fadil, a Syrian citizen and Islamic State operative who was born in Saudi Arabia in 1988. Mr. Fadil entered the country on Jan. 5 and was not on a government list of terrorism suspects, according to Turkish officials.

In addition to the revelation about Mr. Fadil’s refugee status, Turkish officials said they had detained five people in connection with the attack on Tuesday in the popular historic district of Sultanahmet, which killed 10 German tourists and perhaps others.

That the bomber may have entered Turkey with asylum seekers fleeing the war in Syria is likely to increase fears that militants are exploiting the humanitarian crisis to try to carry out attacks against countries allied against the Islamic State.

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The interior minister of Germany met with his counterpart in Turkey after a suicide bombing in Istanbul that killed several German tourists.Published OnJan. 13, 2016CreditImage by Bulent Kilic/Agence France-Presse — Getty Images

And at a time when Turkey had already been under heavy pressure from European officials to stem a huge flow of migrants trying to reach Europe, the bombing is again focusing attention on Turkey’s dilemma in trying to balance national security with its growing immigration stresses.

Turkey recently closed its southern border with Syria to new migrants, and last week new visa regulations were introduced that require Syrians to have a visa to enter the country by air and sea. Prime Minister Ahmet Davutoglu said Wednesday that Turkey would be taking more visible security measures in densely populated areas.

“This person entered Turkey as an ordinary migrant,” Mr. Davutoglu said during a news conference in Istanbul. “Turkey is working to find the true actors behind this attack where Daesh was used as a subcontractor,” he added, using an alternative name for the Islamic State.

Until now, the Islamic State had limited its targets in Turkey to Kurdish and leftist nationalist groups that it linked to its enemies across the border in Syria. It carried out two major suicide attacks last year, including one in the capital, Ankara, in October that killed more than 100 people.

But Tuesday’s bombing was the first major assault aimed at tourists in Turkey. It raised concerns that militants have shifted their tactics to carry out more expansive attacks that include Turkish economic targets.

Analysts say that the attack in Sultanahmet is most likely a response to Turkey’s increased cooperation with the American-led coalition against Islamic State targets in Syria. Turkey is working closely with Washington to seal off its 60-mile border with Syria to prevent the group from using important transit routes.

“I think this attack was first and foremost a warning issued to the Turkish government for taking a more active role as an ally — reluctant and unenthusiastic, but still an ally — of European countries and the U.S. in the fight against ISIS in Syria,” said Erdem Cipa, an assistant professor of history and Near Eastern studies at the University of Michigan. “Hence an act of violence against German nationals on Turkish soil.”

Interior Minister Efkan Ala said Turkey had been detaining dozens of people in antiterrorism raids in recent weeks. He said 3,318 had been detained for possible links to the Islamic State, though it was not immediately clear how many had been prosecuted or released. And he took pains to emphasize that the country was still safe for tourism.

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A man who saw the explosion in Istanbul’s central district of Sultanahmet on Tuesday recounts what he saw.Published OnJan. 12, 2016CreditImage by Osman Orsal/Reuters

The German interior minister, Thomas de Maizière, spoke at a joint news conference here with Mr. Ala, saying there were “no indications that the attack was specifically directed against Germans,” who account for one in six foreign tourists in Turkey.

It was not clear whether there were additional deaths in the attack, Mr. Ala said.

The tourists struck in the attack were part of a group organized by a Berlin-based tour operator. Seven others in the group were injured, Jörg Manthei, a spokesman for Lebenslust Touristik, the travel agency, told DPA, the German news agency.

Citizens of Norway, Peru and South Korea were also among the wounded. On Tuesday, the Peruvian Foreign Ministry said that initial news accounts that a Peruvian man had been killed in the attack were inaccurate; he was wounded, it said.

Within Turkey, the attack immediately added to the backlash against increased immigration.

“Terrorists have been using the migrant path to enter Turkey for years and they have all now set up a presence here,” said Ugur Gunes, a tour operator in the Sultanahmet district. “We can’t expect to take in this many people and for there not to be attacks like this. It’s out of control.”

The country’s Syrian population, now at roughly two million, was also on edge on Wednesday.

“We are worried about the reactions from the Turkish people, as is the case after any attack in Turkey,” said Bilal al-Azaat, 33, a Syrian tour operator in Istanbul.

“In the past, there have been protests in Syrian neighborhoods where people chanted, ‘We don’t want Syrians here.’ ”